This invention relates to a thermite process for producing a pure metal or alloy metal and, more particularly, it relates to a process for efficiently and effectively producing high quality ferroniobium or metallic chromium by an ingenious technique of charging the furnace with a mixture of starting materials.
A thermite process has long been known as a method of producing a metal or alloy by reducing an oxide of the metal or an ore containing an oxide of the metal by another metal such as aluminum or silicon in a powdered state.
The thermite process can be applied for production of a variety of metals and alloys including metallic chromium ferrovanadium, ferroniobium, ferroboron and other ferroalloys.
When any of these alloys or metals is prepared by a conventional thermite process, the starting metallic oxide or ore containing the metallic oxide is crushed and mixed with a reducing agent such as powdered aluminum, to which a slag forming material, an exothermic agent and/or a cooling agent are added if necessary. The mixture is then put in an appropriate furnace and ignited for thermite reaction. The reaction of the mixed materials proceeds exothermically to produce a metal or alloy and a slag under a fused condition, which are then cooled, solidified and separated from each other to obtain the intended metal or alloy.
For producing a metal or alloy by a known thermite process as described above, it is important to keep the temperature of the fused materials formed at or above 2,000.degree. C. in order to effectively separate the intended metal or alloy from the fused slag. However, since the temperature of the thermite reaction is solely dependent on the heat generated by the exothermic thermite reaction, which can significantly vary as a function of the metal involved, the temperature may be too high or too low depending on the type of the metal or alloy to be produced.
If, for example, the amount of the heat generated by the reaction is too large, the process proceeds too vehemently so as to spatter the raw materials and consequently lower the yield. Besides, it may severely erode the lining of the furnace and the security may be threatened. If, on the other hand, the amount of the heat is too small, the yield will also be lowered because the reaction does not proceed at a satisfactory rate and the resultant metal or alloy may contain some of the slag to deteriorate its quality. If the amount of the generated heat is exceptionally too small, the reaction can terminate while the starting materials are only partly fused.
Therefore, it is essential for a thermite process to ensure an appropriate level of heat value, particularly when a metal or alloy is to be produced on an industrial basis and there have been taken specifically designed measures to meet this requirement. Some of the measures include the following.
(1) For an excessively exothermic reaction, the heat generated in the reaction is suppressed by partly replacing the oxide of the metal in the reaction system with the pure metal. Alternatively, a volume of an undersized material of the metal or alloy to be produced (that simply consumes heat to become fused and does not generate heat) is added to suppress the temperature. PA1 (2) For an insufficiently exothermic reaction, an electric furnace is used to supply additional heat to the reaction system. Alternatively, a mixture of powdered aluminum and an easily decomposable oxide that scarcely consumes heat for decomposition is added to the reaction system as an exothermic agent.
The known ordinary thermite process is conducted on a batch basis. A batch of the starting materials are weighed, mixed and then loaded into a furnace in one lot at a time, the amount of the heat to be generated in the reaction being determined by calculating the average exothermic energy of the batch in the reaction system. Of course, meticulous calculations and preliminary experiments are required for determining the exact amount of the starting materials.
However, such a known process is accompanied by problems as described below. Firstly, according to a close observation by the inventors of the present invention on the metallurgical reaction in a known thermite process conducted on a batch basis, the reaction proceeds very slowly in the initial stages, revealing that a heat-insufficiency condition is there, followed by the intermediary stages where the reaction is suddenly accelerated until an excessively high rate of heat generation emerges. When the thermite reaction proceeds with such a profile, there can be cases where the reaction comes to a stand-still somewhere in the initial stages and those where the starting materials are spattered about to lower the yield in the latter part of the reaction while the lining of the furnace becomes very liable to be eroded. Besides, the reaction can proceed very irregularly with the known process so that the resultant metal or alloy may be of inferior quality.